1. Field of the Invention
The present invention relates to the construction of an erasable optical information recording medium and methods of designing its structure such that information signals of high signal quality are overwritten by irradiating a thin film of a phase change material formed on a substrate with a high energy beam such as a laser beam.
2. Description of the Related Art
It is already known as the laser-induced phase change phenomenon that the local heating of a chalcogenide thin film formed on a substrate by irradiating it with a well-focused laser beam produces a reversible change in the optical characteristics, ie. the optical constants: the refractive index and the extinction coefficient, of its microscopic part. In fact, by choosing an appropriate condition for laser irradiation, it is possible to rapidly change the irradiated part from the amorphous phase, which is a comparatively less ordered state of the arrangement of atoms, into the crystal phase, which is a comparatively more ordered state of the arrangement of atoms, and conversely from the crystal phase into the amorphous phase. The information recording that utilizes this phenomenon is called phase change optical recording, and, as an example, optical disk media that are used for high-density information recording have been developed.
Many Te-based alloys of near-eutectic compositions have been examined for quite a long time as candidates of recording thin film materials to be used in phase change optical disk media. For example, Te.sub.81 Ge.sub.15 Sb.sub.2 S.sub.2 (J. Feinleib et al. Appl. Phys. Lett. 18(1971), 254) and Te.sub.60 Ge.sub.4 Sn.sub.11 Au.sub.25 (N. Yamada et al. Proc. SPIE 695(1986), 79) have been the objects of the study. However, thin films of these materials having near-eutectic compositions require at least several hundreds nanoseconds of laser irradiation for crystallization and, therefore, are inadequate for high speed overwriting. In order to solve this problem, it has been recently found that certain stoichiometric compound materials such as GeTe (M. Chen et al. Appl. Phys. Lett. 46(1985), 734), GeSb.sub.2 Te.sub.4 and Ge.sub.2 Sb.sub.2 Te.sub.5 (N. Yamada et al. J. Appl. Phys. 69(1991), 2849), In.sub.3 SbTe.sub.2 (J. Appl. Phys. 64(1988), 1715) and etc. are crystallized by short time laser irradiation of about or less than 100 ns, and these materials are currently attracting considerable attention as recording materials that allow high speed overwriting.
A film structure of an erasable optical disk is a three-layer structure such that a dielectric film layer composed of an oxide or a sulfide or a mixture of these compounds, e.g. a ZnS film, a SiO.sub.2 film, or a ZnS--SiO.sub.2 mixture film, a recording layer composed of a Te-based alloy mentioned above, and a second dielectric film layer are successively deposited on a transparent substrate of resin or glass. A four-layer structure that is made up of the above three-layer structure and an additionally overlaid metallic reflecting layer composed of a Au alloy, an Al alloy, or a Ni--Cr alloy is also known (See, for example, U.S. Pat. No. 4,839,883).
In general, the purpose of adding a metallic reflecting layer is first to enhance the efficiency of light absorption by the recording layer by blocking the passing light so that even a thin recording layer can realize a high degree of light absorption. The second purpose is to enhance the effects of interference of the light reflected from the top surface and the bottom surface of the recording layer by increasing the quantity of the light reflected from the bottom. Since optical phases of the light reflected from the top surface and the light returning from the bottom surface are different, they interfere each other and the interference is either constructive or destructive. Therefore, if the light returning from the bottom surface is designed to be larger, then the interference effects become enhanced, and the difference of the reflectances before and after recording becomes larger. The third purpose is to relieve thermal damage to the recording layer by efficiently diffusing heat produced in the recording layer. In order to fulfill these three objectives, the reflecting layer is required to have sufficiently large reflectance and heat capacity. Therefore, the thickness of the reflecting layer that satisfies these three objectives has usually been chosen to be larger than the one in which the reflectance of the reflecting layer is sufficiently saturated.
It is already known that one of the greatest advantages of phase change optical disks is that an information signal is easily overwritten on it using a single laser beam as its recording means. Specifically, the laser power is modulated, according to the signal level of each point, between the peak level (amorphizing level), which is a power level sufficiently high to melt the irradiated part, and the bias level (crystallizing level), which is lower than the peak level and high enough to raise the temperature above the crystallizing temperature. The information track of the revolving optical disk medium is irradiated with the modulated laser beam so that, even if an old information signal is recorded on the track, a new information signal is recorded while the old signal is erased at the same time (e.g. JP-A-SHO 56-145530). Therefore, phase change optical disks take the same rewriting time as net recording time and are considered to be favorable to the rewriting of video signals and audio signals, which are continual for a long time. In fact, research has been done on degital audio optical disks based on phase-change recording techniques (e.g. K. Nishiuchi et al. Jpn. J. Appl. Phys. 31(1992), 653). Moreover, phase change optical disks do not require magnetic circuit components for recording and playback. Therefore, another advantage is that the recording head can be simplified, and the whole apparatus can be made compact.
However, in the meantime, a problem characteristic of high-speed overwriting recording, which is a great advantage to phase change optical disks, was found. The problem is that the erasability in overwriting operation (called overwrite erasability hereafter) is lower than the erasability in simple erasing by irradiation with an unmodulated laser beam (called DC erasability hereafter) (Nishiuchi et al. Symposium on OPTICAL Memory 1988, p 39). This paper also suggested that the cause of this problem may be due to the fact that the shape of a mark newly recorded by overwriting is distorted through the influence of an old mark recorded before the rewriting. Later studies found that one of the greatest causes of this distortion is the fact that thermal characteristics are different between the amorphous part of a recorded mark and its surrounding crystal part. The crystal part requires absorption of energy equivalent to its latent heat in melting, while the amorphous part does not require absorption of such energy. Therefore, if the same degree of light absorption occurs in the amorphous and crystal parts, the temperature of the amorphous part becomes higher than the crystal part. As a result, if a new amorphous recording is formed on an old amorphous mark having some overlappling part with the old recording mark, then a larger amorphous mark is formed in the overlaping amorphous part than in the part that was an unrecorded crystal part. In short, information of old recording marks remain as a distortions in the shapes of new recorded marks, and the distortions seem to be detected as an residual signal. Recently we reported that, in order to completely eliminate the influence of latent heat, at least 5 percent of .DELTA.A (which will be defined later) is necessary (Yamada et al. Symposium on OPTICAL Memory 1992, p21).
Concerning the above task, we indicated, in JP-A-Hei 1-149238 (EPC SN 88,120,172.7, U.S. Ser. No. 07/276,630), a recording medium in which the light absorption rate A(amo) of a recorded-mark part of the amorphous state and the light absorption rate of an unrecorded part of the crystal state are made equal and a recording medium in which the light absorption rate of the above crystal part is greater than the light absorption rate of the above amorphous part, and we indicated that these media improve the overwrite erasability.
As shown in FIG. 10, we prepared an optical disk medium such that a recording layer 2 inserted between two dielectric layers 3 is stacked over a smooth substrate 1, and a light reflecting layer 4 and a protection layer 5 for covering the same are deposited on top of the three layers. We indicated that the above condition on the light absorption rate A(amo).gtoreq.A(cry) is satisfied by mainly choosing the thickness of the dielectric layers, and the overwrite erasability on this medium is improved compared with prior optical disks such that A(cry)&lt;A(amo). Here the light absorption rate means the quantity of the light absorbed by the recording layer per unit quantity of the incident light, not the physical light absorption coefficient.
Although this method was an epoch-making proposal for improving the overwrite erasability, another problem was left behind. That is the fact that if the difference .DELTA.A=A(cry)-A(amo) is made larger, the difference .DELTA.R=R(cry)-R(amo) of the reflectances between the crystal and the amorphous parts becomes smaller.
FIG. 11 shows a graphical representation of a result obtained from embodiments described in JP-A-Hei 1-149238. As shown in FIG. 11, if the difference .DELTA.A of the light absorption rates are increased to improve the erasability in the recording medium of JP-A-Hei 1-149238, then the difference .DELTA.R of the reflectances decreases in one direction. Therefore, in this medium, the overwrite erasability and the signal amplitude conflict with each other, and it is hard to increase both values at the same time.
Moreover, if .DELTA.A&gt;0 in the media of the embodiments described in JP-A-Hei 1-149238, the absolute amounts of .DELTA.R are not large. While large .DELTA.R=19.1% and .DELTA.R=16.4% were obtained from medium No. 1 having .DELTA.A=-7.4% and medium No. 4 having .DELTA.A=-4% respectively, small .DELTA.R=9.7% and .DELTA.R=11.2% were obtained from medium No. 3 having .DELTA.A=5% and medium No. 6 having .DELTA.A=4.1% respectively. Therefore, small reflectance changes, about 50 to 70% of reflectance changes in media having negative .DELTA.A, were obtained from the media having positive .DELTA.A. In particular, medium No. 3 having the recording layer 40 nm thick had the least reflectance, which was less than 10%. And it was expected that if we tried to increase .DELTA.A more, .DELTA.R would have decreased further.
Since the reflectance change .DELTA.R is a great factor that determines the signal intensity, basically C/N (Carrier-to-noise ratio) decreases as .DELTA.R becomes smaller as shown in FIG. 12. Also, even if .DELTA.R is the same, the C/N decreases as the recording frequency becomes higher, so that the mark pitch becomes finer. It should be remembered that in JP-A-Hei 1-149238 a large C/N of more than 50 dB is obtained when the recorded-mark pitch is over 2 .mu.m (the value calculated from linear velocity 15 m/s and recording frequency 7 MHz). In this mark pitch condition the size (1 .mu.m long if the duty is 50%) of a recorded mark becomes sufficiently large compared with the size (0.9 .mu.m in diameter in half-width) of a laser spot. Therefore, even if .DELTA.R is not so large, a large change in the quantity of light is detected as a size effect, ie. as an integrated value. However, the situation changes when the mark pitch is made finer and recording density is made higher as is currently developed. In this case, the size of a recorded mark (0.65 .mu.m in the present embodiments) becomes equal to or less than the size (0.9 .mu.m in diameter in half-width in the present embodiments) of a laser spot. Then, if .DELTA.R is small, only a small change in the quantity of the reflected light is detected, and a decrease of .DELTA.R has a direct effect of a greatly decreasing C/N. Therefore, if we try to record a signal on an optical disk described in JP-A-Hei 1-149238 at recording density exceeding the one indicated in its description, the C/N decreases inevitably (a detailed description is given later). Under the condition of high-density recording, even the method of JP-A-Hei 1-149238 could not satisfy both a high overwriting C/N and a high overwriting erasability at the same time. In fact, a method of realizing A(cry)&gt;A(amo) and an equivalent value of .DELTA.R as in a structure in which A(cry)&lt;A(amo), eg. over 15%, and a medium that realizes these conditions have not been indicated. Nor has been indicated a method of constructing a medium that realizes a larger .DELTA.A, eg. over 10%, while .DELTA.R is about 10%.
A similar prior proposal tried to equalize light absorption by the crystal and amorphous parts (JP-A-Hei 3-113844). This proposal indicated a medium comprising three layers without a reflecting layer and having a thick recording layer of 80 nm (See JP-A-Hei 3-113844, p. 3, Table 1). However, this prior art was not concerned with a medium having another layer (ie. a reflecting layer) that absorbs light. Considering the fact that light absorption by the recording layer and by the reflecting layer can not be measured separately, this prior example is totally indifferent to the structure of a medium having a reflecting layer.
Further, in the above prior patent JP-A-Hei 3-113844, .DELTA.A is at most 2.1%, and no prospect is shown for obtaining a greater value of .DELTA.A. .DELTA.A and .DELTA.R are in a conflicting relation in this prior art, and table 2 of its specification shows that as .DELTA.A changes from -14% to +2.1%, .DELTA.R sharply declines from 30.3% to 15.7% by about 50%. Therefore, a further attempt to increase .DELTA.A will result in a further decline of .DELTA.A. Thus, if .DELTA.A&gt;0, this prior art does not obtain a value of .DELTA.R equivalent to a value in case .DELTA.A&lt;0. In other words, it can not raise .DELTA.A beyond 2.1% while retaining .DELTA.R at 15% or more.
The distortion in overwriting described above brings a more serious problem in mark-edge detection recording (PWM recording) than in prior PPM recording that detects the peak position of a mark. If the shape of a recorded mark is slightly distorted in PPM recording, detection errors rarely occur because a peak position of a playback signal hardly changes. But a distorted shape of a recorded mark directly brings a detection error in PWM recording because the starting and terminal positions of the mark are detected as a signal. Therefore, if PWM recording is adopted, a medium and a recording method that produce less distortion are necessary.